On Thu, May 29, 2014 at 07:59:20PM -0700, Michael Hawkins
korgpolyex800@... [korgpolyex] wrote:
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> I'll need to see the PIC stuff first before I can make comment. I may try and get a basic functional diagram going so that we can nail down the precise way in which we can plug this into the existing Poly main board and start comparing techniques and component choices.
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> If the PIC VCO is producing a digital output then we shall need a D2A but they can be constructed with some low tolerance resistor ladders = cheap. I have to also check the levels on the Poly to see if the existing TG is producing 0-5 or -5 - +5 volt signals. Either way, we can easily deal with that too. The challenge is to make this CHEAP.
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Don't know why you'd bother with PIC, the toolchain is shite and they are electrically "fragile". If you use a low-end ARM chip you may need to do something to convert between 5V and 3.3V logic, but you'll have enough grunt for all eight voices.
Doing two voices and all four outputs for square and saw ought to be doable in a single atmega 328.
Use a PWM output to generate the "analogue" output instead of an R/2R DAC. Use the counter for the PWM to trigger the sample calculation at around 32kHz and allow it to run asynchronously from the control loop. When the PWM rolls over it'll call an ISR that you use to load the comparison register with the calculated sample value.
Pish easy.
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Gordonjcp MM0YEQ